An internet hiccup on the wrong moment can cause this. And no matter how good your internet is, there's always chance for a hiccup (interference with wireless, router hanging for a second for whatever reason, your ISP experiencing a heavy load, someone starting a download etc.)

Nothing you can do, I'm sure you'll live if it's just 1 day of playing lost.

I'll live alright, without playing Dungeon Defenders and giving them another dollar. It isn't 1 day lost, it's 1 an entire day - every other day. I called my ISP and confirmed that there is no hiccup after reading your comment because I was about to tear them a new one. It's a DD/steam problem, googled it and there are TONS of users that this happens to all the time. Poorly made networking on the games part, maybe they'll fix it one day - people have been making posts since October 2011 about demanding a fix. 14 months later...

do you have "Player 1" over your head, instead of your username? (not character name) Other places to look is on the select a character screen and the bottom, this should be your username, not player 1, and on your forge. If you see player 1, STOP PLAYING it won't register on your account, and anything gained will not be there. Verifying game usually works, but you could try going offline, then back on again. Click "Steam" in the upper left corner, click "go offline" agree to going offline and when steam restarts in offline mode, click "steam" click "go online" and then try again.

that is odd, maybe it is something to do with steam cloud? are you turning DD off abruptly or shutting down steam abruptly?

try waiting for the blue circling wheel on the bottom right to finish spinning before you exit out of the tavern then exit dungeon defenders through the exit button, allow steam some time to sych the DD files before closing steam or the computer

50/50 chance you might be closing it before it can save properly

sorry thats the only idea i can think of atm.

there are a few random bugs my friend also noticed, if he dropped weapons on the tavern floor they will appear on the tavern floor, however as soon as he dropped ultimate weapons they just disappear, he has lost nearly 10 ultimate weapons from this.

if you are playing on ranked your saves are on a trendy server and are saved every time a wave ends you upgrade an item or a hero your antivirus or firewall may be blocking the upload

if you are playing open online or off line your saves are on your pc and it could be windows blocking you from updating the file "administrators rights"or such the files are located in C:\Program Files (x86)\Steam\steamapps\common\Dungeon Defenders\Binaries\Win32 on vista 64bit and are DunDefHeroes.dun and DunDefHeroes.dun.bak if you want to make back ups of them

if you are playing ranked and open you can export you saves from ranked to open but not from open to ranked doing this will always over write your open save

This is on steam's part, sometimes you lose connection to steam even for just a second (noticed it happens more commonly when I go to the website out of the game) and this will drop your current progress from the "cloud". Games that don't have a local save backup (like this one) will be exactly as if you never played since the last save point. This happened to me a lot on Skyrim, but it was safe there because it saves to your pc also. Also, I have comcast and one small fact about cable companies.... They will lie about anything and everything to place any internet issues squarely on you. "It's your router" or "Are your cords frayed" to even "IS it plugged in?"... you have to get aggressive with them to get the truth of "Yes, we did have a very brief outage."

Email them, I've had the same issue and I got everything restored. Stop being a whiner and do something about your problems.

You obviously didn't have the same problems because I did not get anything restored upon email and how could they? It's a Steam problem with their game. Stop being a troll or keep getting reported, your choice.